Electron spectroscopic measurements of band alignment in metal/oxide/semiconductor stacks

Valence and conduction band edges of ultra-thin oxides (SiO₂, HfO₂, Hf_0.7Si_0.3O₂ and Al ₂O₃ grown on silicon) and their shifts upon sequential metallization with three metals (Ru, Ti and Al) have been measured using synchrotron radiation-excited x-ray photoemission, ultra-violet photoemission and inverse photoemission. From these techniques, the offsets between the valence and conduction band edges of the oxides and the metal gate Fermi edge have been directly measured. Upon metallization, consistent shifts of the oxides band edges and core levels are measured, due to the creation of interface dipoles at the metal/oxide interfaces. Using the energy gap, the electron affinity of the oxides and the metal work functions that have been directly measured on these samples, the experimental band offsets are compared to those predicted by the induced gap states model.